Consuming excessive amounts of caffeine may cause restlessness, anxiety and irritability. It may also cause insomnia, headaches and abnormal heart rhythms. Since it affects the neurological system some people experience withdrawal symptoms when they stop using caffeine.
Caffeine is classified by the Food and Drug Administration as GRAS, or generally recognized as safe. There are safety issues related to caffeine use debated and contradicted by many studies, although some of the most reliable are in agreement. In 2010 the American Congress of Obstetricians and Gynecologists disclosed that up to 200 mg of caffeine per day is a safe amount for a pregnant woman. Amounts over 300 mg have been found to increase bone loss in the spines of women over 60. The average amount of caffeine ingested at one time in is estimated to be about 500 mg. Over 10 grams are required for most adults to experience a toxic effect. So clearly moderation is key.
Caffeine is commonly found in nature in the leaves or berries and fruit of many plants. In this environment it can either paralyze and kill some insects eating the plants, or rewards those like bees that pollinate. It is estimated that about 120,000 tons of caffeine stemming from all sources are consumed in the world each day.